Analyzing an inorganic sample via analytical techniques such as x-ray fluorescence (XRF), inductively coupled plasma (ICP), atomic absorption (AA) requires that the sample be specially prepared before analysis. In particular, the sample must often be in the form of a solid, smooth-surface shape, such as that of a disk or bead. In this form, the sample does not exhibit mineralogical, grain-size, or orientation effects that might otherwise skew the analytical results.
A process known as “fusion” can be used to prepare samples for XRF, ICP, and AA. During this process, a powdered sample is dissolved into a solvent, typically a lithium borate flux. The flux is solid at room temperature and therefore must be liquefied. As a consequence, the fusion process is conducted in a furnace/oven, sometimes called a “fluxer”.
To prepare a sample, a precise amount of sample and flux, along with a small amount of a non-wetting agent, are added to a platinum crucible and placed in a fluxer. Upon heating, the flux melts and dissolves the sample. The sample itself never actually melts; it is merely dissolved into the liquefied flux. After complete dissolution, the hot solution is poured into a mold that was also placed in the fluxer. The non-wetting agent prevents the melted flux from sticking to the crucible. Upon cooling, a small, homogeneous glass-like disk or bead of sample results.
Since, as previously indicated, the sample does not melt, the fusion temperature depends almost exclusively on the flux used. Lithium tetraborate (LiT), for example, melts at 920° C. and has the highest melting point of common fluxes. The choice of a particular flux or flux blend depends on sample type. In fact, process temperatures can reach 1200° C. in the fluxer, which poses substantial challenges to the durability of the materials and parts used in the process. At these high temperatures, most materials will burn, melt, rapidly oxidize in the presence of the oxygen, or even be attacked by the halogen gases that are released upon heating.
Fluxers can be driven by gas or electricity. A gas flame provides a quick source of heat, but achieving a precise furnace temperature can be difficult to monitor and adjust. Also, safety concerns over flammable gases in laboratories have prompted many users to switch to electrically powered furnaces, either inductive or resistive.
Although safer than gas, induction furnaces are difficult to calibrate. There have been many reported incidents of overheating, which can lead to damages to the platinumware (e.g., the crucible, etc.) and analytical errors due to evaporation of flux.
The closed compartments of resistive furnaces offer the best temperature stability and accuracy. However, a drawback to resistive furnaces concerns the heating elements. Only a few materials have been used as heating elements for a fusion furnace due to the extreme temperatures required: silicon carbide (SiC), molybdenum disilicide (MoSi2), and iron-chromium-aluminum alloy (FeCrAl).
The electrical characteristics of SiC change over time due to heat exposure. Thus, it is not feasible to replace a single heating element filament in a multi-element configuration. But not replacing a filament when required reduces the life expectancy of the remaining filaments, since they must be driven harder. MoSi2 becomes very brittle upon cooling. This material is also known to react with halogens and degrade prematurely as a consequence thereof. At high temperatures, FeCrAl softens and coil-shaped elements will deform to the point that the turns of a helically wound resistive heating coil (in many prior art designs) will approach one another, leading to element failure due to localized overheating. Additionally, FeCrAl is vulnerable to chemical attack by the flux or halogen compounds. Also, some of these materials can slowly oxidize, reducing the size of the filament and hence its ability to handle power. As a result, the filaments can burn out.
As a consequence, a need remains for improvements in fusion furnace design.